IDAHO FALLS, Idaho -- Potato virus Y infections in Idaho seed have returned to extremely low levels after spiking during the 2011 crop year, according to recently released results of the state's annual winter grow-out.

Of 846 winter grow-out seed lots planted in Brawley, Calif., 38.65 percent contained some level of PVY, which is the lowest amount recorded since the testing program began. By contrast, PVY infection surfaced in more than half of the 2011 lots.

The new data also shows 14.07 percent of lots had at least 2 percent PVY and are ineligible to replant to produce more seed, called re-certification. That's down from 18.84 percent in 2011 but still above 2010's record low of 8.27 percent.

"To me, these are really good numbers," said University of Idaho Extension seed pathologist Phil Nolte. "There have been some very serious increases in PVY in the eastern and central U.S. this year. Their PVY rates are off the scale."

In Maine, for example, Nolte said the industry may have to ease re-certification requirements for PVY to have enough seed. Nolte said several potato states have discussed emulating the program Idaho started in 2007, requiring enzyme-linked immunosorbent assay testing of leaves collected from grow-out plots. ELISA tests confirm the presence of disease in samples based on color changes caused by antibodies. The lab testing, administered by Idaho Crop Improvement Program, is financed through grower certification fees.

Idaho previously relied solely on visual inspections, rendered unreliable by new PVY strains that are less symptomatic and potato varieties, such as Russet Norkotah, that display few visual symptoms.

"The primary reason we've had that reduction in PVY in seed we're sending to commercial growers is because we took that step to test by lab detection rather than visual detection," said Chad Neibaur, a Grace, Idaho, seed grower who serves on the Idaho Crop Improvement Program state board of directors.

Nolte and U of I agricultural economist Chris McIntosh calculated commercial growers lose $16-$18 per acre in reduced yields for every 1 percent of PVY in their seed.

The disease is spread by aphids and can be harbored in volunteer potato plants. Nolte suspects aphid pressure was down last season. Looking forward, he's optimistic a downward trend of PVY infection under the ELISA program will continue next year, as a cold winter has likely kept volunteer spuds in check.

Neibaur plants green manure -- crops raised solely for soil benefits -- after first-generation potato seed to eliminate volunteers. As a rule of thumb, he avoids re-certifying any seed lots found to have half a percent or more of PVY. This year, his seed is well below his personal threshold.

"We had really good numbers-- lots and lots of zeroes," Neibaur said. "Because of those steps (Idaho) probably has the cleanest PVY program of almost anyone in the country."

Nolte also receives an annual grant to harvest additional leaves from grow-out plots, which U of I tests for PVYN, a strain that causes tuber necrosis. He alerts individual growers about positive tests, advising them against re-certifying seed with any amount of PVYN.